This invention relates to a method of making a rechargeable electrolytic battery cell comprising a unitary laminate of polymeric electrode and separator elements. In particular, the invention relates to an economical method of shaping and sizing such a battery cell in a single operation which replaces multiple prior operations, yet ensures proper orientation and size relationships among the respective cell elements.
Versatile rechargeable battery cells, such as lithium-ion intercalation cells, are currently prepared from electrode elements comprising flexible sheets of polymeric composition in which are dispersed finely-divided particulate materials capable of reversibly intercalating lithium ions during battery charge/discharge cycles. Such materials include, as positive electrode components, lithium metal oxide intercalation compounds, e.g., LiCoO.sub.2, LiNiO.sub.2, and LiMn.sub.2 O.sub.4, and, as negative electrode components, carbon materials, such as petroleum cokes and graphites.
Included in the cell structures are flexible electrode-interposed separator layer elements comprising polymers of essentially the same type as employed in the electrode elements, thus facilitating thermal lamination of the element layers to ultimately form the battery composite. Additional cell elements, such as metallic foil electrical current collectors, are also incorporated into the battery structure in a laminating operation.
A laminated battery cell representative of present structures is depicted in FIG. 1 of this specification, and the general process of battery cell fabrication is described in greater detail in U.S. Pat. No. 5,460,904 and its related patent specifications, incorporated herein by reference, which discuss typical compositions and procedures for formulating and laminating composite lithium-ion cells.
In the course of commercial development of the laminated polymeric battery certain requirements for maintaining the integrity and operational condition of these cells have become apparent. For example, while it was originally contemplated that economical mass production of single cell laminated batteries would entail fabrication of a large master laminate body followed by simple perpendicular cutting of the master body, e.g., with common guillotine or similar "punching" equipment, to obtain such batteries of desired size and shape, this anticipated expedient did not prove satisfactory for a number of reasons.
First, the fragile nature of the current collector layer elements resulted in the pressing of electrically conductive fragments of those elements into the laminated polymeric electrode and separator layers with resulting eventual short-circuiting in the cell. Further, the similar action upon the electrode layer compositions themselves forced edge portions of those elements into such close proximity that shorting became inevitable.
Yet an additional disadvantage of the practice was observed in the resultant compression of the separator layer. While this condition contributed somewhat to the noted shorting between electrode layers, more importantly it allowed an overpopulous flow of lithium ions across the shortened edge thickness of separator located between the electrodes, in effect leading to ions bypassing the separator element at that edge and resulting in dangerous plating of metallic lithium at the edge of the negative electrode element during recharging of the cell. In an effort to avoid such a plating condition by providing more ion-intercalating material at the problem site, excess negative electrode composition, usually in the form of an extended electrode layer, was included in the structure. While alleviating somewhat the plating problem, the practice led directly to excessive unproductive electrode material in the cell as a whole, thus increasing the cell weight and degrading specific capacity.
A more direct, yet further uneconomical, practice was then undertaken to appropriately size cell layer elements prior to lamination, including the oversizing of the separator layer to provide sufficient edge distance between electrodes in order to prevent ion bypass and hazardous plating. In this manner the previous cutting problems were eliminated entirely, but there were directly introduced into the fabrication process the disadvantages of multiple cutting and handling of individual cell elements, as well as the greater problem of arranging and maintaining the elements in proper registry during lamination to achieve the desired results.
The cell fabrication method of the present invention, on the other hand, enables the economical use of a master laminate body in that it provides means for avoiding the initial problems of cell element damage and deformation which resulted from punch-cutting and additionally enables the direct formation of sufficient inter-electrode separator edge material to avoid ion bypass and metallic lithium plating during cell recharge cycling. As a result, the invention enables realization of significant savings in time and materials, as well as of the increase in cell efficiencies and capacities which were initially envisioned in the use of laminated polymer cell batteries.